This invention is in the field of data communications, and is more specifically directed to power spectrum control for discrete multitone modulation communications.
Digital Subscriber Line (DSL) technology has become one of the primary technologies in the deployment of high-speed Internet access in the United States and around the world. As is well known in the art, DSL communications are carried out between a central office (CO) location, operated by a telephone company or an Internet service provider, and individual subscribers, using existing telephone “wire” facilities. Typically, some if not all of the length of the loop between the CO and the customer premises equipment (CPE) is implemented by conventional twisted-pair copper telephone wire. Remarkably, modern DSL technology is able to carry out extremely high data rate communications, even over reasonably long lengths (e.g., on the order of 15,000 feet) of twisted-pair wire, and without interfering with conventional voiceband telephone communications.
Modern DSL communications achieve these high data rates through the use of multicarrier modulation (MCM) techniques, also referred to as discrete multitone modulation (DMT), by way of which the data signals are modulated onto frequencies in a relatively wide frequency band (on the order of 1.1 MHz for conventional ADSL, and up to as high as 30 MHz for VDSL), residing well above the telephone voice band, and subdivided into many subchannels. The data symbols modulated onto each subchannel are encoded as points in a complex plane, according to a quadrature amplitude modulation (QAM) constellation. The number of bits per symbol for each subchannel (i.e., the “bit loading”), and thus the number of points in its QAM constellation, is determined according to the signal-to-noise ratio (SNR) at the subchannel frequency, which depends on the transmission channel noise and the signal attenuation at that frequency. For example, relatively noise-free and low attenuation subchannels may communicate data in ten-bit to fifteen-bit symbols, represented by a relatively dense QAM constellation with short distances between points in the constellation. On the other hand, noisy channels may be limited to only two or three bits per symbol, allowing a greater distance between adjacent points in the QAM constellation. High data rates are attained by assigning more bits (i.e., a more dense QAM constellation) to subchannels that have low noise levels and low signal attenuation, while subchannels with poorer SNRs can be loaded with a fewer number of bits, or none at all.
The most popular implementation of DSL is asymmetric DSL (“ADSL”), which follows a frequency-division duplexing (FDD) approach in that “downstream” communications from the telephone company central office (“CO”) to customer premises equipment (“CPE”) are in one frequency band of the spectrum, and “upstream” communications from the CPE to the CO are in another, non-overlapping, frequency band. For example, “downstream” communications (CO to CPE) in modern ADSL occupies 256 subchannels of 4.3125 kHz bandwidth, while upstream communications use 64 such subchannels at lower frequencies than the downstream band (but still above the voice band). ADSL can also be implemented in an echo-cancelled mode, where the downstream frequency band overlaps the upstream frequency band. However, this so-called “overlapped mode” of operation is not widely deployed. In any case, the asymmetry suggested by the acronym “ADSL” refers to the wider and higher-frequency band that is assigned to downstream communications, relative to the narrower, lower-frequency, upstream band. As a result, the ADSL downstream data rate is typically much greater than the upstream data rate, except in those cases in which the loop length is so long that the downstream frequency band becomes mostly unusable. Newer DSL technologies provide higher data rates by variations of the DMT scheme of ADSL. For example, “ADSL2+” extends the data bandwidth to 2.2 MHz using 512 subchannels, and also provides an optional mode in which the upstream data rate can be doubled. Very high bit-rate DSL (“VDSL”) provides extremely high data rates via up to 4096 subchannels, at frequencies extending up to 30 MHz.
In addition to the bit loading and SNR of the subchannels, the available power for DSL transmission is a factor in the actual data rate that can be achieved. Given sufficient power, the signal strength relative to noise can be made high enough for a given subscriber loop that any reasonable data rate can be achieved. But the power levels for communication over a given subscriber loop are in fact limited, primarily because of crosstalk among subscriber loops that are carried over physically adjacent wire facilities. As known in the art, many conventional telephone wire lines are physically located within “bundles” for at least some distance over their length between the CO and the customer premises. This close physical proximity necessarily causes signal crosstalk between physically adjacent conductors in the bundle. The channel characteristics for each DSL user within a bundle thus depend not only on the signal power for that use, but also the signal power of the other users in the bundle and the crosstalk coupling of the signals from those other users. As such, the power level for DSL communications must be limited so that crosstalk among conductor pairs in a bundle can be kept within a reasonable level.
Historically, DSL systems typically consider the problem of crosstalk and power constraints as a “single-user” problem. Modern standards for DSL communication, such as the G.992.1 standard entitled “Asymmetric digital subscriber line (ADSL) transceivers”, promulgated by the International Telecommunications Union, follows this assumption by enforcing a specified power spectral density (PSD) over the entire DSL frequency band for each user. This specified PSD keeps any particular subscriber loop from dominating others in the binder with excessive power, and thus enables reasonable data rates for a large number of subscribers. In addition, this “single-user” solution is easy to implement. However, the enforcing of a specified PSD keeps the overall system from maximizing data rates, by increasing the PSD levels, in those environments in which a higher PSD would not unduly degrade the signal for other users.
But recent advances in the availability of online content, and more widespread deployment of high-speed Internet access, have resulted in increasing demand for higher data rates over DSL connections. In one approach, referred to as very-high bit rate digital subscriber lines (VDSL), the higher data rate is achieved by using higher frequency bands; unfortunately, the crosstalk problem becomes even more severe at higher frequencies. And the widespread popularity of high data rate services are now becoming served through the use of optical fiber facilities for at least part of the length of many subscriber loops, and the deployment of other equipment to extend the reach of DSL service. However, optical network units (ONUs) that interface optical fiber to twisted-pair wire, and remote “DSLAMs” (Digital Subscriber Line Access Multiplexers) that move some of the CO functionality into the field, are notorious sources of additional crosstalk. Worse yet, these remote terminals (RTs) implemented as ONUs and DSLAMs give rise to a so-called “near-far” problem, in that two transmitters (the CO and an ONU, for example) are sourcing interference from different distances from one another; the nearer source of crosstalk, for a given user, will necessarily be stronger than the signal from the more remote source in the loop, thus calling into question the common distance assumption of the fixed PSD limit in conventional DSL. The competing factors of higher data rates and exacerbated crosstalk are thus exerting pressure onto other constraints of DSL technology. As mentioned above, one such constraint is the single-user assumption and the resulting specified PSD limits.
By way of further background, PSD limits on multiple user communications also appear in other technologies. For example, the management of PSD is important in mobile wireless communications, specifically cellular telephone technology. However, many of the problems faced in the mobile wireless context, such as fading signals, moving sources and receivers, and other time-varying factors, are not present in DSL systems. While the channel characteristics of DSL communications may be specific for individual loops, DSL channels do not vary significantly over time. And in contrast to mobile wireless communications, one can attain detailed channel knowledge in DSL loops, and use this knowledge in managing the spectrum.
Accordingly, it has become tempting to attempt to manage the PSD for DSL communications in order to achieve higher data rate communications in this modern context. In one approach, described in Yu et al., “Distributed Multiuser Power Control for Digital Subscriber Lines”, Journal on Selected Areas in Communications, Vol. 20, No. 5 (IEEE; June, 2002), pp. 1105-15, the individual loops in a multi-user DSL environment negotiate power and frequency usage with one another. According to this fully distributed approach, each subscriber loop derives an optimal power allocation and data rate assignment over the subchannels for itself, considering the crosstalk from all other users as noise, and this allocation is successively applied by each of the other users, and iteratively repeated over all users, until convergence. Once this occurs, then each user's total power output is adjusted according to whether the date rate for that user has reached its target data rate; if the data rate is too low, that user increases its total power, or if a user's data rate is well above its target data rate, that user decreases its total power. The “inner loop” of power allocation and data rate assignment is then repeated by all users until convergence, followed by another iteration of total power adjustment relative to data rate. Once all users have converged on an allocation in which they each meet their target data rates, according to this approach, steady-state communications for all users can commence.
In contrast to this fully distributed approach, a centralized power management has the potential to further optimize data rates among multiple users by managing the PSD of each subscriber loop. One such centralized approach is described in Cendrillon et al., “Optimal Multi-user Spectrum Management for Digital Subscriber Lines”, 2004 IEEE International Conference on Communications, Vol. 1 (Jun. 20-24, 2004), pp. 1-5. In this approach, the optimization problem is considered as a Lagrangian, in which a weighted sum of the data rates of two users (a subscriber of interest, and an interferer, for example) are optimized relative to one another, and in a manner that places the appropriate importance on the total power constraints of the users. The weighting factor of the data rates in the weighted sum is modified in an outer loop, with the goal of maximizing the data rate of the interferer while still achieving the target data rate for the subscriber of interest. Inner loops, within this outer loop, determine two Lagrangian multipliers that define the weight of the power constraints of the two users, given the bit loadings of each. According to this approach, a centralized spectrum management center (SMC) is responsible for setting the power spectra for all of the users within the communications network. Optimization of the system in this manner thus requires various parameters (bit loading, channel characteristics, etc.) to be communicated from each user to the SMC for these calculations.